1. Field of the Disclosure
The invention relates to optical transmission systems and, particularly, to long-haul fiber-optic transmission systems.
2. Prior Art
As the complexity and power of optical communication systems increase, the demand for additional diagnostics and safety measures associated with the modern systems is being raised as well. It is rather customary that the power of optical signals transmitted through a fiber reaches the order of Watts. The moment there is a fiber break, which can happen due to a number of external and/or internal reasons, light is no more contained within the fiber and poses a safety hazard to field operators dealing with the maintenance of a transmission system at a variety of deployments. In case of a hazardous situation, it is imperative that the optical power be reduced to acceptable safe power levels and even completely shutdown and subsequently, upon remedying of the problem, controllably restored. The monitoring system recommended by ITU-T includes an optical supervisory channel (OSC) representing a particular interest for the scope of this disclosure. The OSC carries management information about the multi-wavelength optical signal as well as remote conditions at the optical terminal or regeneration site, it is also used for remote software upgrades and user (i.e., network operator) Network Management information.
Some of the known earlier systems incorporate the OSC which is operative to run a battery of diagnostic tests while a transmission system is shut down. Accordingly, the service is interrupted which has far-reaching obvious negative consequences. Other known earlier systems are configured with the OSC associated with the STS-1 SONET standard (STM-1 SDH), which provides for a telephonic service line along the entire length of the transmission system. However, this kind of transmission systems can be effectively used only in a short-haul configuration and, thus, ineffective in a long-haul configuration, which is one of the targets of this disclosure.
Later developed long-haul systems overcome at least some of the above-discussed deficiencies by using the OSC operative to run a battery of diagnostic tests and transmit signals without interrupting the operation of information channels. This is attained by assigning the OSC a separate wavelength that does not interfere with wavelengths of respective information channels in Wavelength Division Multiplexing (WDM)- and Dense WDM (DWDM) transmission systems. Depending on the requirements applied to any given transmission system, the OSC signal is either an in-band or out-of-band signal. The OSC in-band signal propagates within the same band as WDM information signals, but in a different sub-band at wavelengths which rarely, if ever, are used for the WDM signals. In contrast, the out-of-band signal has a wavelength lying beyond the band of interest. In either case, the wavelength of the OSC is selected close enough to the WDM wavelengths to monitor the events leading to the malfunction of the transmission system.
The OSC is typically configured with a transmitter-responder or transponder including optical receiving and transmitting means for communication of data between at least two terminals on the OSC channel. The receiving means may comprise a filter means configured to remove information channels and select the SC, as well as to receive communication data transmitted on OSC. The transmitting means consists of a solid-state laser operating at the OSC wavelength. Both the receiving and transmitting means are coupled to a control means for controlling at least one operational parameter of an optical amplifier and adding communication data therefrom to the OSC.
The OSC can be incorporated, for example, in a short-haul fiber-optic transmission system. Such a system is disclosed in U.S. Pat. No. 7,327,960 and includes a transponder which has a receiver receiving an optical security or supervisory signal (OSS) circulating in the OSC and a solid state transmitter transmitting the OSS at short and mid-range distances. The disclosed transmission system further has a low-power regenerative laser limiting the application of the transponder only to such short distances.
The above discussed limitation was partially overcome by a WDM fiber-optic transmission system utilizing Raman amplifiers spaced along a communication link between terminal nodes and disclosed, for example, in U.S. Pat. No. 7,340,164. The disclosed system is operative to transmit WDM signal between transmitting and receiving nodes in opposite directions via respective communication links. The links each include at least one Raman amplifier which is controllably pumped in response to a control signal generated by control means of a transponder which are coupled to the transponder's receiver. The OSC further includes a transmitting means configured with a solid state laser and operative to transmit communication data on the OSC between the terminals and optical amplifiers. However, the transmission systems configured with Raman amplifiers is not efficient at distances exceeding 150 km.
The efficiency of a long-haul transmission optical system has been improved by introducing an amplifying node which includes an erbium-doped fiber amplifier (EDFA) in addition to Raman amplifiers, as disclosed in US Publication No. 2006/0269287. The disclosed system is provided with a plurality of terminal and amplifying nodes spaced from one another at great distances. The OSC is configured with a receiver which is capable of detecting control data at a lower rate at which the Raman's pump is enabled, and a higher rate which is commenced when the higher power of a Raman amplified signal has been received. A controller and a solid-state laser/transmitter complete the OSC configuration. The utilization of a two-speed configuration of the OSC improves the optimization of data transmission.
However, the above-discussed transmission system is not cost-efficient. Also, in case of failure of both Raman and EDFA lasers, the transmittable data would be lost in both DWDM information and OSC channels, because, at the disclosed OSC rates, the signal would be lower than the detection threshold of the OSC's receiver. Furthermore, the presence of the solid state laser/transmitter of an OSC limits the flexibility of the known transmission systems because the wavelength of the OSS emitted by the laser/transmitter unlikely can be altered.
A need, therefore, exists for a long-haul fiber-optic transmission system overcoming the above-discussed limitations caused by the known transponders so as to reliably and safely transmit a security channel signal data over a distance exceeding 400 km between adjacent amplifying-terminal nodes, even if one or all of the amplifying nodes are completely inoperable.
A further need exists for an optical supervisory channel (OSC) provided with a transponder which is configured with a fiber laser transmitter.
Still a further need exists for a transponder incorporated in the OSC of the long-haul fiber-optic system and configured with a fiber laser/transmitter which outcomes a security optical signals having substantially a single transverse mode.